1. Field of the Invention
This invention relates to a condenser type microphone, and more particularly to a condenser microphone utilizing an electret (permanently electrification) phenomenon.
2. The Prior Art
Heretofore, the electret condenser microphone utilizing variations in capacitance of a condenser comprising an electrically conductive vibration diaphragm and an electrically conductive board having formed thereon an electret film (permanently electrified film) such as FEP (Fluoro Ethylene Propylene) to transform a sound pressure into an electrical signal has been known (see Japanese Utility Model Registration No. 2548543 and Japanese Patent Application Publication No. 11-150795). The electret condenser microphone falls roughly into two categories: the front electret type and the back electret type, based on the positional relationship between the diaphragm and the electret film. Generally, the front electret type is superior in terms of miniaturization whereas the back electret type is superior in terms of performance and cost. The general construction of these two types of electret condenser microphone will be described below.
FIG. 8 is a cross-sectional view illustrating the structure of a microphone 100 which is an example of the electret condenser microphone of the prior art front electret type.
As illustrated in FIG. 8, the microphone 100 includes a capsule 101 in the form of a cylindrical bottom-walled cup made of metal such as aluminum and having an integrally formed bottom wall that is called hereinafter as a front board 101a; an FEP 102 which is an electret film; an annular disc-like spacer 103 formed of an electric insulator such as PET (polyester) or the like; a diaphragm 104 comprising a film 104a such as PET and a metal layer such as nickel vapor-deposited on the film 104a; cylindrical rings 105 and 106 made of an electric conductor such as stainless steel or the like; a base 107 formed of glass epoxy or the like and having a directivity modulating sound aperture 107a formed therethrough and an impedance converting FET (field-effect transistor) 108 and a chip capacitor 109 mounted thereon.
The front board 101a is formed with a receiving sound aperture 101aa which is a through aperture, and an FEP film 102 which is an insulator and has been subjected to polarization treatment to become the electret film is deposited on the entire inner wall surface of the capsule 101 except for its upper folded end portion 101c. The FEP 102, spacer 103, diaphragm 104, ring 105, ring 106 and base 107 are stacked successively in the order named on the FEP film 102 on the inner wall side of the front board 101a. The upper end (or the rear end if the front board is considered as the front end as it faces a sound source) of the capsule 101 is folded inwardly to force an end face of the base 107 towards the front board 101a, which in turn forces the spacer 103, diaphragm 104, ring 105 and ring 106 as a unit towards the front board 101a to be held in place.
In addition, the FET 108 and the chip capacitor 109 are mounted to the inner wall of the base 107, and the output of an electric circuit constituted by these components is electrically connected via through holes 107b, 107c with an output terminal 110a and GND (ground) wiring 110b provided on the outer wall of the base 107. The GND wiring 110b is in turn electrically connected with the capsule 101 at the folded portion 101c. The FET 108 and the chip capacitor 109 are electrically connected with the ring 106 through a wiring (not shown) on the base 107. The ring 106 is electrically connected with the ring 105 which is in turn electrically connected with the vapor-deposited metallic film 104b on the diaphragm 104.
FIG. 9 is a cross-sectional view illustrating the structure of a microphone 200 which is an example of the electret condenser microphone of the prior art back electret type.
As shown in FIG. 9, the microphone 200 includes a capsule 201 in the form of a cylindrical bottom-walled cup made of metal such as aluminum and having an integrally formed front board 201a; cylindrical rings 202 and 208 made of stainless steel or the like; an annular disc-like spacer 203 formed of PET (polyester) or the like; a diaphragm 204 comprising a film 204a such as PET and a metal layer 204b such as nickel vapor-deposited on the film 204a; an FEP 205 which is an electret film subjected to polarization treatment; a plate-like back electrode board 206 formed of stainless steel or the like; a cylindrical insulator holder 207; and a base 209 formed of glass epoxy or the like and having a directivity modulating sound aperture 209a formed therethrough, and an impedance converting FET 210 and a chip capacitor 211 mounted thereon. The front board 201a is formed with receiving sound apertures 201ba, 201bb and 201bc which are through apertures, and the back electrode board 206 has the FEP 205 disposed on the front side surface thereof and is formed with air vents 206a, 206b which are through apertures.
The ring 202, diaphragm 204, and spacer 203 are stacked successively in the order named on the inner side surface of the front board 201a, and the holder 207 and a portion of the back electrode board 206 on the FEP 205 side are disposed on the spacer 203. The ring 208 is further placed on the back electrode board 206, and the base 209 is placed on the holder 207 and ring 208. The thus disposed rings 202, 208, spacer 203, diaphragm 204, back electrode board 206, holder 207 and base 209 are adapted to be forced as a unit towards the front board 201a to be held in place by folding the rear end portion 201c of the capsule 201 inwardly to force an end face of the base 209 towards the front board 201a. 
In addition, the FET 210 and the chip capacitor 211 are mounted to the inner wall of the base 209, and the output of an electric circuit constituted by these components is electrically connected via through holes 209b, 209c with an output terminal 212 and GND wiring 212b provided on the outer surface of the base 209. The GND wiring 212b is in turn electrically connected with the capsule 201. The FET 201 and the chip capacitor 211 are electrically connected with the ring 208 through a wiring (not shown) on the base 209. The ring 208 and the ring 202 are electrically connected with the back electrode board 206 and the front board 201a, respectively.
However, the constructions of these prior art electret condenser microphones of FIGS. 8 and 9 have the drawback that it is liable to deteriorate in sensitivity due to ingress of grit and dust (which will be referred to simply as grit hereinafter) from the outside to the electret film.
For example, in the case of the electret condenser microphone of the front electret type as shown in FIG. 8, the FEP 102 which is an electret film is formed directly on the inner surface of the front board 101a. Consequently, the length of ingress path of grit from the outside of the capsule 101 to the FEP electret film 102 is essentially equal to only the depth of the receiving sound aperture 101aa, that is, the thickness of the front board 101a, so that the likelihood of grit in the outside air to reach the FEP 102 is very high. It is empirically known that if the grit reaches and attaches to the FEP electret film 102, the potential of the capacitor comprising the diaphragm 104 and the front board 101a is reduced, leading to deterioration in sensitivity of the microphone 100.
In the case of the electret condenser microphone of the back electret type as well, if the base 209 is provided with the directivity modulating sound aperture 209a to provide bidirectional properties as shown in FIG. 9, grit is likely to find its way through the directivity modulating sound aperture 209a into the capsule. And if the grit which has thus found its way into the capsule further passes through the air vents 206a, 206b to reach the FEP electret film 102, it may result in deterioration in sensitivity of the microphone, as is the case with the front electret type as discussed above.
It is the microphone disclosed in the above mentioned Japanese Utility Model Registration No. 2548543 that was devised to overcome this drawback. The essential part of this device is shown in FIGS. 10 and 11.
FIGS. 10 and 11 illustrate the improved construction of the receiving sound aperture 101aa in the front board 101a of the prior art microphone 100 shown in FIG. 8. FIG. 10 is a cross-sectional view looking in the direction of arrows C from the sectioned plane indicated in FIG. 11.
This improved through aperture comprises a circular, first recess 101-1 cut in the front board 101a from its front face toward the rear side (from the lower side to the upper side of the front board as viewed in FIG. 10) as indicated by the arrow A to a depth Q equal to approximately half the thickness P of the front board, with several (three, for instance) circumferentially spaced joint portions 101-2 (see FIG. 11) left around the inner periphery of the recess. Then, an annular, second recess 101-3 is cut in the front board 101a from its rear face toward the front side to a depth (P−Q+R) somewhat greater than the remaining half (P−Q) of the front board thickness.
It is to be noted that the first and second recesses 101-3 are sized such that the inner diameter of the second annular recess will touch the outer diameter of the first circular recess 101-1 so that the two recesses will communicate with each other through a slit 101-4 having a depth (R). It is to be understood that when the annular recess 101-3 is formed, the area 101-5 left inward of the annular recess is prevented from being separated from the front board by the joint portions 101-2.
With this improved receiving sound aperture (through aperture) formed as discussed above, grit in the outside air may pass through the first recess 101-1 from the front side and find its way via the slit 101-4 having a depth (R) and then through the second recess 101-3 before reaching the FEP film 102. The ingress of grit may be suppressed by setting the depth R of the slit 101-4 to be small.
However, such improved through aperture must be formed by cutting two recesses into the front board from its opposite sides with a highly precise alignment so as to define a slit 101-4 generally in the middle of the thickness of the front board. This undesirably requires the increased cost of manufacture as well as time and trouble in manufacture. In addition, it is solely the slit 101-4 that contributes to suppressing the ingress of grit, and the length of ingress path of grit remains unchanged, that is, it is essentially equal to the thickness of the front board, so that this improved through aperture has been found insufficient with a limited effect of suppressing the ingress of grit.